This International Standard specifies a method for determining the constant-head water flow capacity within the plane of a geotextile or geotextile-related product, using boundary materials and test conditions of interest. A standard series of test conditions are proposed, involving soil confinement, low hydraulic gradients, seating times and an array of normal loads. NOTE 1 – The results obtained under this test procedure may not compare with those obtained under ISO 12958 – Part 1, even if some of the test conditions are similar. NOTE 2 – Many geosynthetic products can creep under constant load, i.e. see their thickness diminish over time, which may influence their in-plane water flow capacity. Although a seating time typically greater than the one used in ISO 12958-part 1 is used, this test does not cover all creep-related issues for drainage geocomposites. Assessment of long-term flow capacity may require further considerations. NOTE 3 – This procedure may be useful to assess the effect of geotextile intrusion into the drainage core on the transmissivity of a drainage product, using soil from a particular project as a stress-distribution layer in contact with the geotextile. NOTE 4 – There are other test methods which may be more suitable for the characterization of particular drainage products, such as ISO 18325 for Prefabricated Vertical Drains. It is the responsibility of the user to assess the limit of this test procedure and select the appropriate test method, and/or test conditions that adequately reflect the particular needs for his project. NOTE 5 – In this test method, the performance of the product is approached considering soil confinement, service load and service hydraulic gradient, as well as primary creep. However, other field-related issues will affect material long-term performance, such as secondary or tertiary creep, chemical or biological clogging, chemical resistance and durability, installation, backfilling, etc. These issues are covered in separate standards and must be considered while designing with geosynthetics.